The authors suggest the use of a two-tiered classification scheme in order to assess exoplanet habitability. The Earth Similarity Index (ESI), as its name implies, ranks planets based on their similarity to Earth in terms of mass, size, temperature and so on. The Planetary Habitability Index (PHI) ranks planets according to the presence of a stable substrate for life, available energy, appropriate chemistry, and the potential for the planet to hold a liquid solvent. The authors have formulated both indices in such a way that they can be updated as our knowledge advances; this is particularly important for the second tier of the classification scheme, the PHI, since that index requires more information than currently exists for any exoplanet.

The fun bit of the paper, though, is the appearance of a “top-10” list of objects as given by the ESI and the PHI.

The planet with the highest Earth Similarity Index is, of course, Earth. More interestingly, the object with the second-highest Earth Similarity Index is Gliese 581g (Earth has an ESI of 1; Gliese 581g has an ESI of 0.89).

The planet with the highest Planetary Habitability Index is, again no surprise, Earth (which has a PHI of 0.96). Titan, Mars and Europa occupy places 2-4 on the list. The exoplanet with the highest Planetary Habitability Index is, once again, Gliese 581g (with a PHI of 0.45).

So – is Gliese 581g the best place to be looking for alien life (perhaps, as has been suggested, by analysing reflected light from the planet in a search for biomarkers such as the presence of chlorophyll). Maybe. But it’s worth pointing out that it’s not at all certain that Gliese 581g even exists! The Lick-Carnegie Exoplanet Survey ‘discovered’ this exoplanet in September 2010; but the planet did not show up in an analysis of data from the High Accuracy Radial Velocity Planet Searcher. As things stand today, the existence of the planet is unconfirmed.

Perhaps the most habitable exoplanet, as of today, will turn out to have been a mirage.